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pretty-format
Advanced tools
The pretty-format npm package is a JavaScript library that allows you to serialize any JavaScript value into a string with a human-readable format. It is particularly useful for snapshot testing, where you want to compare the expected and actual output of your test cases.
Pretty-printing of basic JavaScript types
This feature allows you to convert basic JavaScript types like objects, arrays, strings, numbers, etc., into a nicely formatted string.
const prettyFormat = require('pretty-format');
const value = { foo: 'bar', baz: 42 };
console.log(prettyFormat(value));
Customizing output with plugins
pretty-format supports plugins that can be used to customize the output for specific types of values, such as React elements.
const prettyFormat = require('pretty-format');
const ReactElementPlugin = require('pretty-format/plugins/ReactElement');
const reactElement = <div>Hello World</div>;
console.log(prettyFormat(reactElement, { plugins: [ReactElementPlugin] }));
Minimizing diff output
By using pretty-format in combination with a diffing library like jest-diff, you can minimize the output of diffs to make them easier to read and understand.
const prettyFormat = require('pretty-format');
const diff = require('jest-diff');
const oldValue = { a: 'old', b: 'values' };
const newValue = { a: 'new', b: 'values' };
const difference = diff(prettyFormat(oldValue), prettyFormat(newValue));
console.log(difference);
Chalk is a popular npm package for styling terminal strings. While it doesn't serialize objects, it can be used in conjunction with pretty-format to colorize the output, enhancing readability.
Util is a core Node.js module that provides a method called 'inspect' for printing objects in a readable format. It is similar to pretty-format but is built into Node.js and does not support plugins.
js-beautify is an npm package that can format HTML, CSS, and JavaScript code. It is more focused on formatting code rather than serializing arbitrary JavaScript values like pretty-format.
Stringify any JavaScript value.
$ yarn add pretty-format
const prettyFormat = require('pretty-format'); // CommonJS
import prettyFormat from 'pretty-format'; // ES2015 modules
const val = {object: {}};
val.circularReference = val;
val[Symbol('foo')] = 'foo';
val.map = new Map([['prop', 'value']]);
val.array = [-0, Infinity, NaN];
console.log(prettyFormat(val));
/*
Object {
"array": Array [
-0,
Infinity,
NaN,
],
"circularReference": [Circular],
"map": Map {
"prop" => "value",
},
"object": Object {},
Symbol(foo): "foo",
}
*/
function onClick() {}
console.log(prettyFormat(onClick));
/*
[Function onClick]
*/
const options = {
printFunctionName: false,
};
console.log(prettyFormat(onClick, options));
/*
[Function]
*/
key | type | default | description |
---|---|---|---|
callToJSON | boolean | true | call toJSON method (if it exists) on objects |
escapeRegex | boolean | false | escape special characters in regular expressions |
escapeString | boolean | true | escape special characters in strings |
highlight | boolean | false | highlight syntax with colors in terminal (some plugins) |
indent | number | 2 | spaces in each level of indentation |
maxDepth | number | Infinity | levels to print in arrays, objects, elements, and so on |
min | boolean | false | minimize added space: no indentation nor line breaks |
plugins | array | [] | plugins to serialize application-specific data types |
printFunctionName | boolean | true | include or omit the name of a function |
theme | object | colors to highlight syntax in terminal |
Property values of theme
are from ansi-styles colors
const DEFAULT_THEME = {
comment: 'gray',
content: 'reset',
prop: 'yellow',
tag: 'cyan',
value: 'green',
};
The pretty-format
package provides some built-in plugins, including:
ReactElement
for elements from react
ReactTestComponent
for test objects from react-test-renderer
// CommonJS
const prettyFormat = require('pretty-format');
const ReactElement = prettyFormat.plugins.ReactElement;
const ReactTestComponent = prettyFormat.plugins.ReactTestComponent;
const React = require('react');
const renderer = require('react-test-renderer');
// ES2015 modules and destructuring assignment
import prettyFormat from 'pretty-format';
const {ReactElement, ReactTestComponent} = prettyFormat.plugins;
import React from 'react';
import renderer from 'react-test-renderer';
const onClick = () => {};
const element = React.createElement('button', {onClick}, 'Hello World');
const formatted1 = prettyFormat(element, {
plugins: [ReactElement],
printFunctionName: false,
});
const formatted2 = prettyFormat(renderer.create(element).toJSON(), {
plugins: [ReactTestComponent],
printFunctionName: false,
});
/*
<button
onClick=[Function]
>
Hello World
</button>
*/
For snapshot tests, Jest uses pretty-format
with options that include some of its built-in plugins. For this purpose, plugins are also known as snapshot serializers.
To serialize application-specific data types, you can add modules to devDependencies
of a project, and then:
In an individual test file, you can add a module as follows. It precedes any modules from Jest configuration.
import serializer from 'my-serializer-module';
expect.addSnapshotSerializer(serializer);
// tests which have `expect(value).toMatchSnapshot()` assertions
For all test files, you can specify modules in Jest configuration. They precede built-in plugins for React, HTML, and Immutable.js data types. For example, in a package.json
file:
{
"jest": {
"snapshotSerializers": ["my-serializer-module"]
}
}
A plugin is a JavaScript object.
If options
has a plugins
array: for the first plugin whose test(val)
method returns a truthy value, then prettyFormat(val, options)
returns the result from either:
serialize(val, …)
method of the improved interface (available in version 21 or later)print(val, …)
method of the original interface (if plugin does not have serialize
method)Write test
so it can receive val
argument of any type. To serialize objects which have certain properties, then a guarded expression like val != null && …
or more concise val && …
prevents the following errors:
TypeError: Cannot read property 'whatever' of null
TypeError: Cannot read property 'whatever' of undefined
For example, test
method of built-in ReactElement
plugin:
const elementSymbol = Symbol.for('react.element');
const test = val => val && val.$$typeof === elementSymbol;
Pay attention to efficiency in test
because pretty-format
calls it often.
The improved interface is available in version 21 or later.
Write serialize
to return a string, given the arguments:
val
which “passed the test”config
object: derived from options
indentation
string: concatenate to indent
from config
depth
number: compare to maxDepth
from config
refs
array: find circular references in objectsprinter
callback function: serialize childrenkey | type | description |
---|---|---|
callToJSON | boolean | call toJSON method (if it exists) on objects |
colors | Object | escape codes for colors to highlight syntax |
escapeRegex | boolean | escape special characters in regular expressions |
escapeString | boolean | escape special characters in strings |
indent | string | spaces in each level of indentation |
maxDepth | number | levels to print in arrays, objects, elements, and so on |
min | boolean | minimize added space: no indentation nor line breaks |
plugins | array | plugins to serialize application-specific data types |
printFunctionName | boolean | include or omit the name of a function |
spacingInner | strong | spacing to separate items in a list |
spacingOuter | strong | spacing to enclose a list of items |
Each property of colors
in config
corresponds to a property of theme
in options
:
tag
)colors
is a object with open
and close
properties whose values are escape codes from ansi-styles for the color value in theme
(for example, 'cyan'
)Some properties in config
are derived from min
in options
:
spacingInner
and spacingOuter
are newline if min
is false
spacingInner
is space and spacingOuter
is empty string if min
is true
This plugin is a pattern you can apply to serialize composite data types. Of course, pretty-format
does not need a plugin to serialize arrays :)
// We reused more code when we factored out a function for child items
// that is independent of depth, name, and enclosing punctuation (see below).
const SEPARATOR = ',';
function serializeItems(items, config, indentation, depth, refs, printer) {
if (items.length === 0) {
return '';
}
const indentationItems = indentation + config.indent;
return (
config.spacingOuter +
items
.map(
item =>
indentationItems +
printer(item, config, indentationItems, depth, refs), // callback
)
.join(SEPARATOR + config.spacingInner) +
(config.min ? '' : SEPARATOR) + // following the last item
config.spacingOuter +
indentation
);
}
const plugin = {
test(val) {
return Array.isArray(val);
},
serialize(array, config, indentation, depth, refs, printer) {
const name = array.constructor.name;
return ++depth > config.maxDepth
? '[' + name + ']'
: (config.min ? '' : name + ' ') +
'[' +
serializeItems(array, config, indentation, depth, refs, printer) +
']';
},
};
const val = {
filter: 'completed',
items: [
{
text: 'Write test',
completed: true,
},
{
text: 'Write serialize',
completed: true,
},
],
};
console.log(
prettyFormat(val, {
plugins: [plugin],
}),
);
/*
Object {
"filter": "completed",
"items": Array [
Object {
"completed": true,
"text": "Write test",
},
Object {
"completed": true,
"text": "Write serialize",
},
],
}
*/
console.log(
prettyFormat(val, {
indent: 4,
plugins: [plugin],
}),
);
/*
Object {
"filter": "completed",
"items": Array [
Object {
"completed": true,
"text": "Write test",
},
Object {
"completed": true,
"text": "Write serialize",
},
],
}
*/
console.log(
prettyFormat(val, {
maxDepth: 1,
plugins: [plugin],
}),
);
/*
Object {
"filter": "completed",
"items": [Array],
}
*/
console.log(
prettyFormat(val, {
min: true,
plugins: [plugin],
}),
);
/*
{"filter": "completed", "items": [{"completed": true, "text": "Write test"}, {"completed": true, "text": "Write serialize"}]}
*/
The original interface is adequate for plugins:
highlight
or min
depth
or refs
in recursive traversal, andWrite print
to return a string, given the arguments:
val
which “passed the test”printer(valChild)
callback function: serialize childrenindenter(lines)
callback function: indent lines at the next levelconfig
object: derived from options
colors
object: derived from options
The 3 properties of config
are min
in options
and:
spacing
and edgeSpacing
are newline if min
is false
spacing
is space and edgeSpacing
is empty string if min
is true
Each property of colors
corresponds to a property of theme
in options
:
tag
)colors
is a object with open
and close
properties whose values are escape codes from ansi-styles for the color value in theme
(for example, 'cyan'
)This plugin prints functions with the number of named arguments excluding rest argument.
const plugin = {
print(val) {
return `[Function ${val.name || 'anonymous'} ${val.length}]`;
},
test(val) {
return typeof val === 'function';
},
};
const val = {
onClick(event) {},
render() {},
};
prettyFormat(val, {
plugins: [plugin],
});
/*
Object {
"onClick": [Function onClick 1],
"render": [Function render 0],
}
*/
prettyFormat(val);
/*
Object {
"onClick": [Function onClick],
"render": [Function render],
}
*/
This plugin ignores the printFunctionName
option. That limitation of the original print
interface is a reason to use the improved serialize
interface, described above.
prettyFormat(val, {
plugins: [pluginOld],
printFunctionName: false,
});
/*
Object {
"onClick": [Function onClick 1],
"render": [Function render 0],
}
*/
prettyFormat(val, {
printFunctionName: false,
});
/*
Object {
"onClick": [Function],
"render": [Function],
}
*/
24.9.0
[expect]
Highlight substring differences when matcher fails, part 1 (#8448)[expect]
Highlight substring differences when matcher fails, part 2 (#8528)[expect]
Improve report when mock-spy matcher fails, part 1 (#8640)[expect]
Improve report when mock-spy matcher fails, part 2 (#8649)[expect]
Improve report when mock-spy matcher fails, part 3 (#8697)[expect]
Improve report when mock-spy matcher fails, part 4 (#8710)[expect]
Throw matcher error when received cannot be jasmine spy (#8747)[expect]
Improve report when negative CalledWith assertion fails (#8755)[expect]
Improve report when positive CalledWith assertion fails (#8771)[expect]
Display equal values for ReturnedWith similar to CalledWith (#8791)[expect, jest-snapshot]
Change color from green for some args in matcher hints (#8812)[jest-snapshot]
Highlight substring differences when matcher fails, part 3 (#8569)[jest-core]
Improve report when snapshots are obsolete (#8665)[jest-cli]
Improve chai support (with detailed output, to match jest exceptions) (#8454)[*]
Manage the global timeout with --testTimeout
command line argument. (#8456)[pretty-format]
Render custom displayName of memoized components (#8546)[jest-validate]
Allow maxWorkers
as part of the jest.config.js
(#8565)[jest-runtime]
Allow passing configuration objects to transformers (#7288)[@jest/core, @jest/test-sequencer]
Support async sort in custom testSequencer
(#8642)[jest-runtime, @jest/fake-timers]
Add jest.advanceTimersToNextTimer
(#8713)[@jest-transform]
Extract transforming require logic within jest-core
into @jest-transform
(#8756)[jest-matcher-utils]
Add color options to matcherHint
(#8795)[jest-circus/jest-jasmine2]
Give clearer output for Node assert errors (#8792)[jest-runner]
Export all types in the type signature of jest-runner
(#8825)[jest-cli]
Detect side-effect only imports when running --onlyChanged
or --changedSince
(#8670)[jest-cli]
Allow --maxWorkers
to work with % input again (#8565)[babel-plugin-jest-hoist]
Expand list of whitelisted globals in global mocks (#8429)[jest-core]
Make watch plugin initialization errors look nice (#8422)[jest-snapshot]
Prevent inline snapshots from drifting when inline snapshots are updated (#8492)[jest-haste-map]
Don't throw on missing mapper in Node crawler (#8558)[jest-core]
Fix incorrect passWithNoTests
warning (#8595)[jest-snapshots]
Fix test retries that contain snapshots (#8629)[jest-mock]
Fix incorrect assignments when restoring mocks in instances where they originally didn't exist (#8631)[expect]
Fix stack overflow when matching objects with circular references (#8687)[jest-haste-map]
Workaround a node >=12.5.0 bug that causes the process not to exit after tests have completed and cancerous memory growth (#8787)[docs]
Replace FlowType with TypeScript in CONTRIBUTING.MD code conventions[jest-leak-detector]
remove code repeat (#8438)[docs]
Add example to jest.requireActual
(#8482)[docs]
Add example to jest.mock
for mocking ES6 modules with the factory
parameter (#8550)[docs]
Add information about using jest.doMock
with ES6 imports (#8573)[docs]
Fix variable name in custom-matcher-api code example (#8582)[docs]
Fix example used in custom environment docs (#8617)[docs]
Updated react tutorial to refer to new package of react-testing-library (@testing-library/react) (#8753)[docs]
Updated imports of react-testing-library to @testing-library/react in website (#8757)[jest-core]
Add getVersion
(moved from jest-cli
) (#8706)[docs]
Fix MockFunctions example that was using toContain instead of toContainEqual (#8765)[*]
Make sure copyright header comment includes license (#8783)[*]
Check copyright and license as one joined substring (#8815)[docs]
Fix WatchPlugins jestHooks.shouldRunTestSuite
example that receives an object (#8784)[*]
Enforce LF line endings (#8809)[pretty-format]
Delete obsolete link and simplify structure in README (#8824)[docs]
Fix broken transform link on webpack page (#9155)FAQs
Stringify any JavaScript value.
The npm package pretty-format receives a total of 81,920,967 weekly downloads. As such, pretty-format popularity was classified as popular.
We found that pretty-format demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 3 open source maintainers collaborating on the project.
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